The role of phosphorous in animal nutrition is well recognized. Phosphorus is a critical component of the skeleton, nucleic acids, cell membranes and some vitamins. Though phosphorous is essential for the health of animals, not all phosphorous in feed is bioavailable.
Phytates are the major form of phosphorous in seeds. For example, phytate represents about 60-80% of total phosphorous in corn and soybean. When seed-based diets are fed to non-ruminants, the consumed phytic acid forms salts with several important mineral nutrients, such as potassium, calcium, and iron, and also binds proteins in the intestinal tract. These phytate complexes cannot be metabolized by monogastric animals and are excreted, effectively acting as anti-nutritional factors by reducing the bioavailability of dietary phosphorous and minerals. Phytate-bound phosphorous in animal excreta also has a negative environmental impact, contributing to surface and ground water pollution.
There have been two major approaches to reducing the negative nutritional and environmental impacts of phytate in seed. The first involves post-harvest interventions, which increase the cost and processing time of feed. Post-harvest processing technologies remove phytic acid by fermentation or by the addition of compounds, such as phytases.
The second is a genetic approach. One genetic approach involves developing crop germplasm with heritable reductions in seed phytic acid. While some variability for phytic acid was observed, there was no change in non-phytate phosphorous. Further, only 2% of the observed variation in phytic acid was heritable, whereas 98% of the variation was attributed to environmental factors. Another genetic approach involves selecting low phytate lines from a mutagenized population to produce germplasm. Most mutant lines exhibit a loss of function and are presumably blocked in the phytic acid biosynthetic pathway; therefore, low phytic acid accumulation will likely be a recessive trait. In certain cases, this approach has revealed that homozygosity for substantially reduced phytate can be lethal. Another genetic approach is transgenic technology, which has been used to increase phytase levels in plants. These transgenic plant tissues or seed have been used as dietary supplements.
The biosynthetic route leading to phytate is complex and not completely understood, and it has been proposed that the production of phytic acid occurs by one of two possible pathways. One possible pathway involves the sequential phosphorylation of Ins(3)P or myo-inositol, leading to the production of phytic acid. Another possible pathway involves hydrolysis of phosphatidylinositol 4,5-bisphosphate by phospholipase C, followed by the phosphorylation of Ins(1,4,5)P3 by inositol phosphate kinases. In developing plant seeds, accumulating evidence favors the sequential phosphorylation pathway. Such evidence includes studies of the Lpa2 gene, a gene encoding a maize inositol phosphate kinase which has multiple kinase activities. The Lpa2 gene has been cloned, and the lpa2 mutation has been shown to impair phytic acid synthesis. Mutant lpa2 seeds accumulate myo-inositol and inositol phosphate intermediates.
The maize low phytic acid 1 mutant (lpa1) was isolated from an EMS-mutagenized population in the early 1990s by USDA scientists. However, the original lpa1-1 allele was previously known to have a phenotype of up to 15% loss of seed dry weight, which could translate into a yield drag if the lpa1-1 mutant was used in product development. Since the discovery of lpa1, the gene responsible for the lpa1 mutation has been sought for two reasons: 1) the mutant has a phenotype of low phytic acid and high available phosphorus in grain which makes it useful in animal feeding and phosphorus waste management; and 2) the lpa1 mutant does not accumulate myo-inositol phosphate intermediates, indicating that mutation in this locus impairs a critical step in the phytic acid biosynthesis pathway which was previously uncharacterized.
Based on the foregoing, there exists the need to improve the nutritional content of plants, particularly corn and soybean, by increasing non-phytate phosphorous and reducing seed phytate. Accordingly, it is desirable to isolate and characterize the Lpa1 gene in order to place the expression of this gene under tight control so as to produce plants which have reduced seed phytate and increased non-phytate phosphorus.